1. Field of the Invention
The present invention relates to a focus condition detecting device for a camera which detects the focusing condition of the objective lens through measurement of the light rays coming from an object to be photographed (hereinafter referred to as object light rays) and having passed through the objective lens.
2. Description of the Prior Art
Many focus detecting devices have been proposed wherein object light rays having passed through first and second areas of the objective lens that are symmetric with each other with respect to the optical axis of the objective lens, are reconcentrated or re-converged by a pair of re-imaging lenses to form images after once concentrated or converged by the objective lens, and the relative positions of the two images are detected to determine the amount and direction of defocus of the object image formed by the objective lens, or the amount and direction of deviation of the position of the object image from a predetermined focal plane (whether the object image is in front of or in the rear of the predetermined focal point, i.e. whether a front focus or a rear focus condition is attained). A typical optical system of such focus detecting devices has a construction as shown in FIG. 1. The optical system includes a condenser lens 6 disposed on or in the back of a predetermined focal plane 4 which in turn is to the rear of an objective lens 2. To the rear of the condenser lens 6 are disposed a pair of re-imaging or relay lenses 8 and 10. A pair of linesensors 12 and 14 which have CCDs as the light detectors, are disposed on the respective predetermined focal planes of the re-imaging lenses. The images formed by re-imaging lenses 8 and 10 on linesensors 12 and 14 are hereinafter referred to as re-images of the object image formed by the objective lens 2. The re-images are nearer or closer to the optical axis 18 and to each other as shown in FIG. 2 when the object image is formed in front of the predetermined focal plane i.e. in the case of front focus. In contrast, the re-images are distant from the optical axis 18 in the case of rear focus. When the objective lens 2 is in an in-focus condition, the distance between two corresponding points of the two re-images has a given value determined by the construction of the optical system of the focus detecting device. Accordingly, the focus condition can be basically determined by detecting the distance of the two re-images based on the outputs of the linesensors. The following method has been known as one of the methods for detecting the distance between the two re-images.
With reference to FIG. 3, sensors 12 and 14 are respectively composed of ten and sixteen photodiode cells 21, a1 to a10 and b1 to b16. Assume for the convenience that the reference characters assigned to each cell also represents the output level of the cell. If consecutive ten cells are to be taken from the sensor 14, seven sets B1, B2 . . . B7 can be made. The in-focus condition can be detected by determining on which one of the seven sets is formed the re-image that best coincides with the re-image on the sensor 12. For example, if the re-image on the set B1 of the sensor 14 best coincides with the re-image on the sensor 12, that is, if the relationship of a1=b1, a2=b2 . . . a10=b10 is found between outputs of corresponding cells of the two sets a1 to a10 and b1 to b10, the total sum S1 of the absolute values of the differences between outputs of corresponding cells will be the critically as follows: EQU S1=.vertline.a1-b1.vertline.+.vertline.a2-b2.vertline.+ . . . .vertline.a10-b10.vertline. . . . =0 (1)
Thus, the values S1 is smaller than any other values calculated in the same way for the sets other than B1. In other words, the value of S1 is the smallest of the sums of the absolute values of the differences calculated in the same way for all the sets. To find the minimum value, the calculations as given by the formula (1) is made for all the sets and the sums obtained from the calculations are compared with one another. If it is detected that the re-image on the set B1 best coincides with the image on the set A1, then it is determined whether the set B1 is at the predetermined standard position, nearer or closer to the optical axis than the standard position or more distant than the same, as well as what is the amount of deviation of the set B1 from the standard position. From the result, it is determined whether the objective lens is in the in-focus, front focus or rear focus condition and also the amount and direction of driving required for the objective lens to be brought into the in-focus condition is calculated. To make such determination and calculation, it is necessary that a particular cell of the linesensor is set at a particular position in the optical system of the focus detecting device and that the ordinal numbers of the other cells with reference to the particular cell can be identified in the program for the determination and calculation. Although in the above explanation two separated linesensors 12 and 14 are used, in practice a part of a single linesensor is used as the linesensor 12 and another part of the same is used as the linesensor 14. Accordingly the aforementioned particular cell may be a particular one of (generally the one at the center of) a single linesensor including the parts for the linesensor 12 and 14. Further, the generality will not be lost even if a particular point on the optical axis is regarded as the particular position in the optical system of the focus detecting device. Thus, it is necessary in practice that a cell at a particular address or ordinal number in a linesensor is exactly positioned on the optical axis of the optical system of a focus detecting device.
However, no practical method has been proposed for exactly positioning and fixing a linesensor relative to the optical system of a focus detecting device. The cells are aligned at the pitch of about 30 .mu.m in a CCD linesensor and it is required that the linesensor should be positioned with the accuracty of .+-.15 .mu.m. One method to be thought primary may be that every part of the focus detecting device should be made with high accuracty such that the linesensor may be positioned exactly only by assembling such parts. However, such a method requires high accuracy in making the parts and is not fit for mass production because the focus detecting device made through this method will be expensive. A method to assemble the device with the aid of jigs requires a standard plane or point to be provided on some portion of parts and is substantially the same as the above method in requiring accuracy in making the parts. Another method may be thought wherein a linesensor is fixed at the position of a focus detecting device where an image of a slit is projected on a particular cell of the linesensor. In this method, the relative positions of the slit and the focus detecting device must be determined such that the slit is exactly positioned on the optical axis of the focus detecting device. However, no proper method for the purpose has been proposed.